The present invention relates generally to gas turbine engines, and, more specifically, to turbines therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in turbine stages which power the compressor and a shaft that typically drives a fan in an aircraft turbofan engine application.
A high pressure turbine (HPT) directly follows the combustor and receives the hottest gases therefrom from which energy is initially extracted. A low pressure turbine (LPT) follows the HPT and extracts additional energy from the gases.
As energy is extracted from the gases in the various turbine stages, the velocity and pressure distributions correspondingly vary, which in turn requires correspondingly different aerodynamic profiles of the turbine stator vanes and rotor blades. The size of the vanes and blades typically increases in the downstream direction for providing more surface area to extract energy from the combustion gases as the pressure thereof decreases.
The velocity of the gases also decreases as energy is extracted and the flowpath area increases, which in turn leads to changes in the span and thickness aspect ratios of the vanes and blades and corresponding camber thereof.
Fundamental to turbine efficiency is the aerodynamic performance of the individual turbine airfoils as the combustion gases are split along the leading edges thereof for corresponding flow along the generally concave pressure side of the airfoil and the generally convex suction side thereof. Differential pressure is effected between the opposite airfoil sides, and aerodynamic contour or camber of the airfoil is optimized for maximizing differential pressure without undesirable flow separation of the gases over the suction side.
The turbine flowpath is defined circumferentially between adjacent airfoils as well as radially between inner and outer flowpath surfaces. For the turbine nozzle, inner and outer bands integral with the vanes bound the flow. And for the turbine blades, radially inner platforms and radially outer shrouds bound the combustion gases.
A particular problem affecting turbine efficiency is the generation of undesirable vortices as the combustion gases are split along the airfoil leading edges near a flow boundary, such as the radially inner blade platforms. Two horseshoe vortices flow downstream on opposite sides of each airfoil and create undesirable turbulence in the flow. This turbulence can increase platform heating. And, migration of the vortices radially outwardly can decrease turbine efficiency.
The outer and inner flowpath boundaries in the typical gas turbine engine may vary in contour or axial profile, but are axisymmetrical with constant diameter or radius from the axial centerline axis of the engine at each axial plane. The blade platforms, for example, are therefore axisymmetric with uniform circumferential curvature from their upstream forward ends to their downstream aft ends notwithstanding any axial inclination or slope thereof.
In previous turbine developments, it is known to selectively contour the flowpath boundaries to minimize the adverse affects of the horseshoe vortices. However, due to the complex three dimensional (3D) configuration of the turbine stages and the correspondingly complex 3D distributions of the velocity, pressure, and temperature of the combustion gases contouring of the flowpath boundaries is equally complex and is directly affected by the specific design of the specific turbine stage.
Accordingly, known flowpath contouring is highly specific to specific turbine stages and is not readily transferable to different stages whose efficiency and performance could instead be degraded.
Adding to the complexity of design and environment are the special flow fields around the radially outer tips of the turbine blades which rotate at high speed inside a surrounding stationary shroud during operation. Combustion gases which leak over the airfoil tips in the required clearance between the tips and shroud perform little, if any, useful work.
Modern turbine blade design typically incorporates squealer tip ribs which are small radial extensions of the pressure and suction sides of the airfoil from leading to trailing edge. The tip ribs are typically rectangular in cross section and spaced transversely or circumferentially apart to define an open tip cavity atop the airfoil which has an integral tip floor that encloses the typically hollow airfoil and the internal cooling circuit therein.
The small tip ribs provide sacrificial material in the event of a tip rub to protect the tip floor and internal cooling circuit from undesirable damage. The tip ribs increase the complexity of the combustion gas flow field introducing local secondary fields which affect turbine efficiency, flow leakage, and tip cooling.
The primary flow direction of the combustion gases is in the axially downstream direction in the flow passages defined between adjacent blades. The axial flow stream also varies along the radial direction from root to tip of each airfoil, and is significantly affected by the horseshoe vortices. And, these axial and radial flow variations are further compounded over the airfoil tip where the combustion gases leak between the pressure and suction sides of each airfoil.
Accordingly, it is desired to provide a turbine rotor blade having an improved configuration for improving turbine performance and efficiency.